1. Field of the Invention
The invention relates to a graytone mask and a manufacturing method thereof.
2. Description of the Related Art
In recent years, attempts have been made to reduce the number of mask sheets by using graytone masks in the field of large-sized LCD masks (as set forth in the monthly FPD Intelligence, May, 1999).
As shown in FIG. 5A, such a graytone mask has an opaque part 1, a transmission part 2 and a graytone part 3. The graytone part 3 corresponds to an area in which there is formed an opaque pattern 3a of below or equal to the resolution limit of an exposure apparatus for a large-sized LCD using the graytone mask and is designed to selectively change the thickness of a photoresist film by decreasing the light transmitted through this area so as to decrease the amount of irradiation due to the area. Normally, the opaque part 1 and the opaque pattern 3a are formed with films that are made of the same material such as chromium (Cr) or a chromium compound and have the same thickness.
The resolution limit of the exposure apparatus for the large-sized LCD using the graytone mask is about 3 μm in the case of an exposure apparatus of a stepper type and about 4 μm in the case of an exposure apparatus of a mirror projection type. Consequently, the space width of a transmission part 3b in the graytone part of FIG. 5A is set at less than 3 μm and the line width of the opaque pattern 3a of below or equal to the resolution limit of the exposure apparatus is set at less than 3 μm, for example. When the exposure apparatus for the large-sized LCD is used for light exposure, as the exposure light transmitted through the graytone part 3 as a whole is deficient in the amount of light exposure, positive photoresists are left on a substrate though the thickness of the positive photoresists exposed to light via the graytone part 3 solely decreases. More specifically, there arises a difference in solubility of resists in developing liquid between parts corresponding to the ordinary opaque part 1 and to the graytone part because of difference in the amount of light exposure and this results in, as shown in FIG. 5B, making a part 1′ corresponding to the ordinary opaque part 1 as thick as about 1.3 μm, making a part 3′ corresponding to the graytone part 3 as thick as about 0.3 μm and making a part corresponding to the transmission part 2 a part 2′ without resists, for example. A first etching of a substrate as a workpiece is carried out in the part 2′ without the resists so as to remove the resists in the thin part 3′ corresponding to the graytone part 3 by ashing and the like and by carrying out a second etching of this part, the etching process is performed with one mask instead of two masks as conventionally used in order to reduce the number of masks for use.
In order to form a graytone section in the graytone mask in the form of a fine pattern having a size equal to the resolution limit of a large LCD aligner using a graytone mask, a minute line-and-space pattern is ideally given a pitch of ±2 μm or thereabouts (about one-half this pitch; that is, 1 μm or thereabouts, becomes a fine light transmission section). A fine pattern must be processed with an accuracy of ±0.2 μm or thereabouts. However, in terms of the accuracy achieved by the current large mask for use in manufacturing an LCD, this represents very rigorous accuracy.
A current large mask automatic imperfection inspection system encounters great difficulty in finding an imperfection at a pitch pattern of 2 μm (particularly, an imperfection in the edge of a pattern). Further, great difficulty is encountered also in carrying out inspection of a fine pattern with an accuracy of ±0.2 μm or thereabouts. Further, since the graytone section is formed from a fine pattern, the volume of data required to be prepared becomes enormous. When the volume of data has exceeded the capacity of an aligner or a data converter (or format converter) associated with the aligner, aligning may become impossible. Specifically, data pertaining to the graytone sections 3 shown in, e.g., FIG. 6B, must be prepared so as to avoid an opaque section and a light transmission section for preventing occurrence of an overlap between the opaque section 1 and the light transmission section 2 shown in FIG. 6A. As a result, preparation of data becomes complicated. In addition, data pertaining to the graytone sections 3 show complicated geometries defined along the opaque section and the light transmission section. Hence, the volume of data pertaining to the graytone sections becomes enormous, and hence the volume of data pertaining to merged data shown in FIG. 6C also becomes enormous.
In a certain known graytone mask, the thickness of a chromium single layer film laid on a transparent substrate is changed locally. In the mask, an area having a large thickness is taken as an opaque section; an area having an intermediate thickness is taken as a graytone section; and an area having no thickness is taken as a light transmission section. However, a chromium film has low transmissivity (i.e., a high opaque characteristic), and hence a small thickness enables achievement of 0% transmissivity. For this reason, difficulty is encountered in half-etching the chromium single layer film so as to achieve a predetermined intermediate transmissivity in the area having an intermediate thickness. Therefore, there has been proposed a graytone mask in which the thickness of a chromium compound single layer film laid on a transparent substrate is partially changed (as described in Japanese Patent Application Laid-Open No. 49410/1995). In this case, since an area of the chromium compound single layer film where a 0% transmissivity is achieved assumes a thickness as large as about 4000 angstroms, half-etching of the film such that a predetermined intermediate transmissivity is achieved in an area having an intermediate thickness becomes easier than in the case of the chromium single layer film. However, since the thickness is excessive, an aspect ratio (i.e., a ratio of a pattern size to a height) becomes high, consequently deteriorating the pattern geometry and accuracy of an opaque section and resulting in a longer etching time. In fact, rigorous control of a thickness during half-etching operation entails difficulty. Thus, thickness control involves practical difficulty.